Batteries are commonly used as energy sources. Typically, a battery includes a negative electrode and a positive electrode. The negative and positive electrodes are often disposed in an electrolytic medium. During discharge of a battery, chemical reactions occur wherein an active positive electrode material is reduced and active negative electrode material is oxidized. During the reactions, electrons flow from the negative electrode to the positive electrode through a load, and ions in the electrolytic medium flow between the electrodes. To prevent direct reaction of the active positive electrode material and the active negative electrode material, the electrodes are mechanically and electrically isolated from each other by a separator.
One type of battery is a lead-acid battery. In a lead acid battery, lead is usually an active negative electrode material, and lead dioxide is usually an active positive electrode material. (In a lead-acid battery, the electrodes are often referred to as “plates”.) Generally, lead acid batteries also contain sulfuric acid, which serves as an electrolyte and participates in the chemical reactions.
A mat comprised of glass fibers may serve as a separator. The glass mat separator has a critical role in electrolyte filling. Any change in the physical properties of this material can drastically change the quality of the filled and formed battery. The separator structure, degree of compression and fiber composition have a significant influence on how well an unfilled element will accept electrolyte. While high levels of compression are desirable for extended life, this may make the filling and formation process more difficult. When the separator is compressed, the pore size is reduced, along with more restricted access to void, or empty volume in the separator. This will make the filling process more difficult.
When electrolyte is added to the battery, the ideal situation is that all areas are wetted as much as possible by the same amount and concentration of acid so that there is perfectly uniform distribution of electrolyte throughout the plate stack when the filling process is completed. This ideal situation is difficult or impossible to achieve in practice, as there is a dynamic competition between the separator and the plate surfaces for the electrolyte. As the electrolyte penetrates into the plate stack, it is held up by the separator (the capillary forces tend to hold the electrolyte rather strongly), and at the same time the electrolyte is depleted by the exothermic reaction of the sulfuric acid with the plate by the simple chemical reaction of PbO+H2SO4=>PbSO4+H2O. As the liquid front penetrates deeper into the stack it becomes more dilute and also gets hotter, due to the exothermic reaction with the lead oxide. One of the likely threats is the formation of hydration shorts/dendrites. As the acid reacts with the lead oxide, the sulfuric acid electrolyte becomes progressively more dilute. Lead sulfate is relatively soluble in the hot electrolyte with low acid strength and near neutral pH, and dissolved lead sulfate will diffuse into the separator. This will hasten the formation of lead dendrites and/or hydration shorts. A short circuit may develop and be detected during formation, or more subtly the battery will fail prematurely in service due to the formation of lead dendrites through the separator structure. If the filling process is poor or incomplete, individual cells may also have “dry areas” after filling. These poorly wetted areas may include no acid or water (completely dry), dilute acid or just water. These dry areas will slowly become wetted during and after formation, but significant grid corrosion may result due to unformed active material forcing all of the current to flow through the grid only.
During discharge, the sulfuric acid in the electrolyte is consumed and water is produced, diluting the acid concentration and causing the specific gravity of the electrolyte to decrease. During charging, formation of lead and lead oxide in the negative and positive plates, respectively, results in release of pure sulfuric acid. Due to its high specific gravity, the pure sulfuric acid tends to settle toward the bottom (or “stratify”) in the electrolyte, a phenomenon known as “acid stratification”. In a stratified battery, electrolyte concentrates at the bottom, starving the upper part of the cell. The light acid on top limits plate activation, promotes corrosion and reduces the performance, while the high acid concentration on the bottom makes the battery appear more charged than it is and artificially raises the open-circuit voltage.
Unfortunately, design or materials changes that improve battery performance and/or life, e.g., separators that exhibit resistance to acid stratification, generally may also tend to make proper filling more difficult.